Elemental iodine is a valuable chemical having many industrial and medicinal applications. Much attention is therefore focused on the recovery of iodine from various sources, either as a primary product or as a by-product of other industrial processes. Iodine recovery is generally carried out by physical and/or chemical manipulation of an aqueous solution containing soluble ions of iodine. Exemplary solutions include leaching solutions used in nitrate extraction, and brine solutions. The term "brine" in this context includes industrial and naturally occurring salt solutions containing iodine in various salt forms. Exemplary brines are seawater and natural brines such as those associated with petroleum deposits and with solution mining of salt domes. In addition, the secondary recovery of iodine is also carried out, for example, in recycling X-ray photographic plates.
While brines are a significant domestic source of iodine, nitrate ore processing supplies the majority of iodine consumed worldwide and is obtained as a byproduct of nitrate ore processing. Large deposits of sodium nitrate ore are found in Chile, and hence the nitrate produced is commonly referred to as Chilean nitrate. The nitrate ore, known as "caliche", is extracted with a leaching solution to yield soluble sodium nitrate. The nitrate is then recovered from solution by crystallization, and the liquid remaining after removal of the nitrate crystals is called the "mother liquor". Most nitrate ore processing plants use the Guggenheim process, in which crushed ore is leached in a series of vats by countercurrent extraction. After leaching, the ore is washed in countercurrent stages with water to remove residual nitrate solution. The liquid used for extraction in the leaching step consists of a mixture of the mother liquor from the nitrate crystallization and the wash liquid or wash "liquor" recovered from the washing step. Sulfate salts such as CaSO.sub.4, MgSO.sub.4 and K.sub.2 S0.sub.4 are also added to the extraction liquid and serve to break-up the double salt NaNO.sub.3.Na.sub.2 SO.sub.4 and thereby increase nitrate yield. This double salt forms part of the nitrate in the caliche but is difficult to extract if not treated with sulfates.
Iodine, in the form of sodium iodate (NaIO.sub.3), is co-extracted with the nitrate during leaching and remains in the mother liquor after the nitrate is crystallized out. Because the mother liquor is recycled back to the leaching step, the iodate level in the mother liquor builds up and will reach unacceptable levels if not removed. To avoid this, a portion of the mother liquor is drawn off from nitrate processing when the iodate reaches a target concentration (e.g., about 6 g/liter) and is treated to remove the iodate. After iodate removal, the treated mother liquor portion is returned to the nitrate leaching step and serves to reduce the overall iodate concentration in the mother liquor.
Removal and recovery of the iodate in the form of iodine is a two step process. In a first step, the iodate is reduced to iodide by reaction with sodium bisulfite: EQU 2NaIO.sub.3 +6NaHSO.sub.3 .fwdarw.2NaI+3Na.sub.2 SO.sub.4 +3H.sub.2 SO.sub. 4
After reaction, a product mixture of sodium iodide and by-product sodium sulfate and sulfuric acid is produced.
In a second step, the product mixture from the first step is reacted with additional mother liquor from the nitrate process. Iodate in the additional mother liquor, in the presence of the sulfuric acid produced in the first step, reacts with the iodide to form elemental iodine as a precipitate: EQU 5NaI+NaIO.sub.3 +3H.sub.2 SO.sub.4 .fwdarw.3I.sub.2 +3Na.sub.2 SO.sub.4 +3H.sub.2 O
After removal of the precipitated iodine, the treated mother liquor portion is neutralized prior to its return to the nitrate process.
The recovery of iodine from brine employs one of two processes. In a first process, silver iodide is precipitated via addition of silver nitrate to the brine. The silver iodide is filtered and then treated with iron to form ferrous iodide. Chlorine is then used to liberate elemental iodine, which is then recovered by filtration.
In a second process, iodine is recovered as a co-product of natural brine. The brine is acidified with sulfuric acid and treated with a slight excess of chlorine to form elemental iodine as a precipitate. The brine is then pumped to a denuding tower where the iodine is entrained in a countercurrent stream of air. The iodine is then absorbed in a solution of hydriodic and sulfuric acids. This solution is then treated with sulfur dioxide to reduce the iodine to (additional) hydriodic acid. A portion of the hydriodic acid is diverted to a reactor where it is reacted with chlorine to form precipitated elemental iodine, which is recovered by filtration and then further processed.
Regardless of whether the iodine source is nitrate processing, brine or secondary recovery, a critical step is the recovery of precipitated iodine from the corresponding aqueous suspension. In the Chilean nitrate process, known procedures for recovering precipitated iodine are marginal at best. Typically, after the second reaction step in which the iodine is precipitated, the treated nitrate mother liquor is conveyed to a filter press where the iodine crystals are separated and recovered manually. This manual operation is labor intensive, time consuming, and hazardous to worker health. At ambient temperatures the vapor pressure of iodine is sufficiently high to permit the release of significant amounts of iodine into the surrounding environment. Workers engaged in the breakdown, cleaning and reassembly of the filter presses are particularly vulnerable, since these functions are accompanied by open exposure to iodine. Furthermore, the release of iodine into the environment is a significant source of pollution.
A further disadvantage of the currently-used filter presses are that a significant amount of mother liquor may remain in the precipitate after its removal from the filter press, requiring further process steps.
The filtration and recovery of iodine from brine processing suffers from many of the same disadvantages. That is, there may be considerable exposure of workers to iodine and the filtration process may be inefficient.
For the above reasons, there is a need in the art for a method for filtering and recovering iodine precipitate which is more efficient, less labor intensive, and minimizes exposure of workers and the environment to iodine.